Electronic oven



y 2, 1957 J. E. STAATS 3,317,700

ELECTRONIC OVEN Filed Sept. 29,- 1964 2 Sheets-Sheet 1 INVENTOR. L/AMES E. STAATS ATTORNEY J. E. STAATS ELECTRONIC OVEN May 2, 1967 2 Sheets-Sheet 2 Filed Sept. 29, 1964 ww ldwl III 8 T A nm 5 E M J ATTORNEY United States Patent 3,317,700 ELECTRONIC OVEN James Edward Staats, Louisville, Ky., assignor to General Electric Company, a corporation of New York Filed Sept. 29, 1964, Ser. No. 400,070 6 Claims. (Cl. 219--10.55)

This invention relates to electronic ovens and has particular relation to a tunable transmission line utilized in electronic ovens.

Electronic ovens of conventional design include a radio frequency generator for producing radio frequency electromagnetic energy whichis applied to a cavity containing food for heating the food. The generator ordinarily comprises a magnetron tube having output terminals which are coupled to the cavity by a suitable transmission line.

In the application of electronic ovens to commercial use, it has been found that customer requirements differ according to the principal use desired for the oven. For example, one customer may use the oven primarily for thawing of frozen food while another may use the oven primarily for reheating a precooked specialty item. It has been observed that various types of foods capable of being heated by electromagnetic energy have widely varying impedance properties. As a result, if the load impedance, which includes the food impedance and the line impedance, is matched with the generator output impedance for one type of food, it will ordinarily not be matched for another type of food. When a mismatch exists, part of the energy applied to the load is reflected back to the generator resulting in a lower efliciency than would be realized for a matched impedance condition for which no appreciable energy is reflected back to the generator. Consequently, a fixed line impedance does not provide optimum performance for all oven customers.

It is thus seen to be desirable that an adjustable means be provided to permit variation of the transmission line impedance for obtaining an impedance match for various types of foods so that such foods can be heated with high efiiciency. The adjustable means must be of simple and low cost design so as not to add appreciably to the cost of the electronic oven, and should be mounted in an accessible position so as to be readily adjustable and to be positively maintained in its adjusted position.

It is therefore a primary object of the invention to provide a novel and improved electronic oven capable of heating various types of foods with optimum efficiency.

It is another object of the invention to provide an electronic oven including a radio frequency generator coupled to a food cavity by a transmission line which includes a reliable and low cost adjustable tuning means for adjusting the impedance of the transmission line.

It is a further object of the invention to provide an electronic oven including an elongated coaxial transmission line for coupling electromagnetic energy from a radio frequency generator to a food cavity and which includes an adjustable tuning means for adjusting the impedance of the transmission line.

In carrying out the invention in one form, the electronic oven includes a food heating cavity coupled to a radio frequency generator by an elongated coaxial transmission line which delivers electromagnetic energy from the generator to the cavity. The transmission line includes a hollow outer conductor coupled to one output terminal of the generator, and an inner conductor extending through the outer conductor and coupled to the other terminal of the generator. To permit adjustment of the line impedance a slug of suitable metallic or dielectric material is slidably mounted on the inner conductor and is adjustably retained thereon by suitable fastening means.

Other objects and advantages of the invention will become apparent from the following description taken in 3,317,700 Patented May 2, 1967 ice conjunction with the accompanying drawings in which:

FIGURE 1 is a view in front elevation of an electronic oven embodying the present invention;

FIGURE 2 is a view in top plan of the oven of FIGURE 1 with the top wall removed and with a portion of the upper transmission line broken away to expose the adjustable tuning member;

FIGURE 3 is a view in side elevation of the oven of FIGS. 1 and 2 with a side wall removed and with parts broken away;

FIGURE 4 is a view in elevation showing the radio frequency generator and the transmission line coupled thereto with parts broken away and with parts shown in section; and

FIGURE 5 is a perspective view showing a part of the transmission line and the adjustable tuning member.

Referring now to the drawing, FIGURES 1-3 show an electronic oven represented generally by the numeral 10 and comprising a housing or casing 11 having front and rear walls 13 and 14 respectively connected by side walls 15 and 16 and top and bottom walls 18 and 19 respectively. The casing contains a cavity 21 (FIG. 3) which is defined by the side wall 15, a side plate 23, a rear plate 24 and a top plate 25 all within the casing. The cavity 21 includes an open end at its front formed by an opening in the front wall 13. In order to expose and conceal the cavity 21 a door 23 is mounted on the front wall 13 by a hinge structure 29 for movement between an open position wherein the door extends horizontally to expose the cavity, and a closed position wherein the door extends vertically as illustrated in the drawing to close the cavity. A handle 31 is fixed to the door to facilitate the opening and closing thereof.

The oven 10 is designed to heat food in the cavity 21 and for this purpose includes a radio frequency generator represented generally by the numeral 33 for producing electromagnetic energy which has a frequency of about 915 megacycles per second and which is coupled to cavity 21 by a suitable transmission line represented generally by the numeral 34. The generator 33 may be of any suitable construction and is preferably a magnetron tube which is suitably supported within the casing 11 behind the cavity 21 or toward the right hand side of the cavity as viewed in FIGURES 2 and 3. As shown in FIG. 4, the generator 33 includes a housing comprising a pair of generally U-shaped sections 35 and 36 preferably formed of soft iron and secured together by screws 37 and 39. The housing is secured to mounting chanels 41 and 42 on the bottom wall 19 of the casing 11.

In the specific embodiment illustrated the generator 33 includes a cathode and cathode heater terminal 43 and a cathode terminal 44 which are exposed for connection to a suitable source of alternating voltage. The generator 33 includes a pair of output terminals at the lower end thereof as viewed in FIGURE 4. One of the output terminals is in the form of a cylindricalmagnetic pole piece 46 formed of soft iron and connected to the anode structure (not shown). The other output terminal comprises a conductive stud 48 also connected to the anode structure (not shown). The pole piece 46 is surrounded by a pair of axially spaced electroconductive field windings 50 and 51 which are connected in series with each other and with the anode structure by conductors including conductors 55 and 56. The conductor 56 is connected to the housing sections 35 and 36 by means of the screw 39 and the conductor 55 is adapted to be connected to the positive terminal of a source of direct current voltage (not shown). When energized, the windings 50 and 51 produce an axial magnetic field as is understood by those skilled in the art. A radio frequency generator suitable for use in the oven of the present invention is disclosed in the application Serial Number 105,983, filed April 27, 1961 by James E. Staats and assigned to the assignee of the present invention.

In order to couple the output of generator 33 to the cavity 21 the transmission line 34 is provided. As best shown in FIGURE 3, the line 34 includes three main sections 60, 62 and 63, the lower section 60 being coupled to the output terminals of generator 33 and extending horizontally along the bottom wall 19 of the casing 11, the upper section 62 being coupled to the cavity 21 and extending horizontally along the top wall 18 of the casing, and the section 63 extending vertically and interconnecting the sections 60 and 62. The transmission line is preferably of the coaxial type and in the embodiment illustrated, the section 60 includes an outer hollow conductor 65 and an inner solid conductor 67 extending through the conductor 65. In a similar manner, the section 62 includes an outer hollow conductor 69 and an inner solid conductor 71 extending through the conductor 69. The conductors 65, 67, 69 and 71 preferably have transverse cross section areas of generally rectangular configuration.

As shown in FIGURES 3 and 4, the section 63 includes an outer hollow cylindrical conductor or sleeve 73 surrounding a cylindrical hollow insulating tube 74 preferably formed of Teflon. The tube 74 surrounds a hollow cylindrical conductor 75 which is electrically and mechanically connected to the conductor 65 of the section 60. The outer sleeve 73 also surrounds the lower end of a hollow cylindrical conductor 76 which is electrically and mechanically connected to the outer conductor 69 of the section 62 and which is spaced axially from the conductor 75. A central solid lower inner conductor 77 is surrounded by conductor 75 and is electrically connected to the conductor 67 of section'60. The conductor 77 is det-achably connected to and insulated from a solid inner conductor 78 by a connecting filter unit 79. The conductor 78 is electrically connected to the conductor 71 of section 62. With the described arrangement, the conductors '65 and 67 are capacitively coupled respectively to the conductors 76 and 78 and the high direct current voltages on the conductors 65 and 67 are therefore not applied to conductors 76 and 78. The conductor 76 is clamped to a collar on the conductor 69 by means of a clamp 80 whereas the sleeve 73 is clamped to conductor 76 by means of a clamp '81. A transmission line suitable for use in the present invention is disclosed and claimed in application Ser. No. 135,- 582, filed September 1, 1961 by L. H. Fitzmayer and assigned to the assignee of the present invention, now Patent 3,172,987.

In order to couple the section 60 of the line 34 of the output terminals of the generator 33 the line 60 is provided with a coupling section comprising a hollow cylindrical conductor 82 which is mechanically and electrically connected to the conductor 65 and which extends into the hollow pole piece 46 in frictional engagement therewith. A metallic seal 83 surrounds the conductor 82 and is positioned between the pole piece 46 and the conductor 65. The inner conductor 67 of section 60 is insulated from outer conductor 65 by a spacer 88 and is electrically connected to a conductive fixture 84 which surrounds a portion of -a metallic stud 87 which contains a cavity receiving the output terminal 48.

In order to couple electromagnetic energy from the transmission line 34 to the cavity 21 there is provided :an antenna structure 89 which is coupled to the section 62 of the line 34. The antenna structure 89 forms no part of the present invention and may be of any suitable construction. An antenna structure suitable for use in the present invention is disclosed and claimed in application Ser. No. 296,622, filed July 22, 1963, by James E. Staats, now Patent 3,221,132.

In accordance with the present invention a readily adjustable tuning means is provided for effecting a match between the impedance of the load and the impedance of the generator 33. The load consists of a food load within the cavity 21 and also the transmission line 34 leading from the generator 33 to the cavity 21. In the present invention the adjustable tuning means is associated with the transmission line 34, and in the illustrated embodiment the adjustable means comprises a pair of conductive mmebers and 102 secured respectively to the inner conductors 67 and 71 of the line sections 60 and 62 for sliding adjustment relative to such conductors. The members 100 and 102 are of similar configuration and a description of one will suflice. As best shown in FIGURE 5, the member 100 is in the form of a generally flat plate having a central section 103 overlying the conductor 67. The member 100 is adjustably secured to the conductor 67 in any suitable manner and for this purpose includes at its ends separate pairs of opposed bent portions with the portions 104 and 105 being positioned at one end of the section 103 and with the portion 106 and 107 positioned at the other end of the section 103. The portions 104107 serve to frictionally mount the member 100 on the conductor 67 for sliding movement relative thereto. The members 100 and 102 may be formed of either conductive material or of a dielectric material such as Teflon.

It is observed that when the members 100 and 102 are adjusted along their respective inner conductors, a substantial variation of the impedance of the transmission line between the generator and the cavity is realized. Such impedance variation may be effected over a substantial range by adjusting the members 100 and 102. A very fine adjustment is obtainable inasmuch as the transmission line 34 is intentionally designed to have a substantial length between the generator and the cavity. Each of the members 100 and 102 provides means for reflecting waves in the line 34. It has been observed that the dimensions of the members determine the magnitude of the reflections, and that the positions of the members longitudinally of their associated line sections determine the phase relationship between the reflected waves and the principal wave traveling from the generator to the cavity. Therefore, the ratio of voltage to current of the composite wave, which is comprised of the reflected and principal waves, may be varied by adjustment of the members to thereby vary the impedance presented to the composite wave.

In practice, a factory adjustment is made by placing the customers principal food type in the cavity and then adjusting one or both of the members 100 and 102 by means of a dielectric rod inserted through the end of the appropriate line section. 'These sections have end caps which are removable to allow insertion of the rod. During the adjustment the frequency and current of the generator are measured and the members are adjusted until the values of the frequency and current indicate a matched load or favorable impedance.

Provision of the adjustable means in the form of members 100 and 102 permits a very close matching of the impedance of the load and of the generator 33 which results in a very high efliciency of operation. The adjustment is readily accomplished and the members 100 and 102 are of low cost and compact construction and thus do not add appreciably to the overall cost and size of the oven.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electronic oven, means defining a food heating cavity, a high frequency energy source having a pair of output terminals, and a tunable transmission line coupling the source to the cavity, said transmission line including a hollow outer conductor coupled to one terminal of said source, an inner solid conductor extending through the outer conductor and coupled to the other terminal of said source, and a tuning member movably mounted on the outer surface of said inner conductor to adjust the impedance of the transmission line between said source and said cavity, said tuning member being outside said cavity intermediate the ends of said inner conductor and spaced from said outer conductor.

2. An oven as defined in claim 1 wherein said inner and outer conductors have cross sectional areas of substantially rectangular configuration, said tuning member comprising a generally flat plate supported on said inner conductor.

3. In an electronic oven, means defining a food heating cavity, a high frequency energy source having a pair of output terminals, and a tunable transmission line coupling the source to the cavity, said transmission line including a pair of electrically connected vertically spaced and horizontally extending sections each comprising a hollow outer conductor and an inner solid conductor extending through the outer conductor, the conductors of the lower one of said sections being coupled to said output terminals, and the conductors of the upper one of said sections being coupled to said cavity, and a pair of tuning members each movably mounted on the outer surface of a separate one of said inner conductors to adjust the impedance of the transmission line between said source and said cavity, each of said tuning members being outside said cavity intermediate the end-s of the associated inner conductor and spaced from the associated outer conductor.

4. An oven as defined in claim 3 wherein the inner and outer conductors of each section have cross sectional areas of substantially rectangular configuration, each tuning member comprising a generally flat plate supported on its associated inner conductor.

5. An oven as defined in claim 4 wherein each tuning member includes a pair of tabs at each end thereof bent over to partially surround and engage the associated inner conductor.

6. In an electronic oven, a casing having front, rear, top, and bottom walls and including a food heating cavity adjacent the front wall, a door at the front wall movable to expose and conceal said cavity, a high frequency energy source behind said cavity adjacent the rear wall having a pair of output terminals adjacent the bottom wall, and a tunable transmission line coupling said source to said cavity, said transmission line including a first horizontally extending section adjacent the bottom wall comprising a first hollow outer conductor of generally rectangular cross section coupled to one of said output terminals, and a first inner solid conductor of generally rectangular 'cross section extending through said first outer conductor and coupled to the other of said output terminals, said transmission line including further a second horizontally extending section adjacent the top wall and electrically connected to said first section, said second section including a second hollow outer conductor of generally rectangular cross section, and a second inner solid conductor of generally rectangular cross section extending through said second outer conductor, an antenna in said cavity coupled to said second section, and a pair of tuning members each slidingly mounted on the outer surface of a separate one of said first and second inner conductors to adjust the impedance of the transmission line between said source and said cavity, each of said tuning members being outside said cavity intermediate the ends of the associated inner conductor and spaced from the associated outer conductor. 

1. IN AN ELECTRONIC OVEN, MEANS DEFINING A FOOD HEATING CAVITY, A HIGH FREQUENCY ENERGY SOURCE HAVING A PAIR OF OUTPUT TERMINALS, AND A TUNABLE TRANSMISSION LINE COUPLING THE SOURCE TO THE CAVITY, SAID TRANSMISSION LINE INCLUDING A HOLLOW OUTER CONDUTOR COUPLED TO ONE TERMINAL OF SAID SOURCE, AN INNER SOLID CONDUCTOR EXTENDING THROUGH THE OUTER CONDUCTOR AND COUPLED TO THE OTHER TERMINAL OF SAID SOURCE, AND A TUNING MEMBER MOVABLY MOUNTED ON THE OUTER SURFACE OF SAID INNER CONDUCTOR TO ADJUST THE IMPEDANCE OF THE TRANSMISSION LINE BETWEEN SAID SOURCE AND SAID CAVITY, SAID TUNING MEMBER BEING OUTSIDE SAID CAVITY INTERMEDIATE THE ENDS OF SAID INNER CONDUCTOR AND SPACED FROM SAID OUTER CONDUCTOR. 